There is a need within the graphic arts industry to print large format halftone images, generally in the range of 20.times.24 inches or greater, at extremely high resolutions at reasonable scanning speeds. Drums scanners that have been used in the past for printing large format images tend to require several minutes to produce an image. For example, a drum scanner available from Rudolf Hell Gmbh, model Hell DC 300, uses an argon laser split into six beams to simultaneously write on the external surface of a printing drum. Each of the six beams is independently modulated with an acousto-optic modulator which typically limits the bandwidth of each writing channel to a few megapixels per second. The drum scanner uses a water-cooled argon laser with an external modulator and associated optics which also makes the system very bulky. The scanner takes seven minutes to print a 20.times.24 inch image.
Hologon scanning systems have been proposed that overcome some of the disadvantages of the previously described scanner. The hologon scanning systems typically utilize a laser diode to generate a laser writing beam. The laser diode is either internally modulated or external modulation is provided by an acousto-optic or electro-optic modulator device. The write beam generated by the laser diode is deflected to scan across a target by a grating disc that is rotated by a motor. A laser diode is much more efficient and less bulky than the water-cooled argon laser employed in the Hell DC 300 scanner. In addition, modulation of the laser diode can be accomplished at pixel rates of gigapixels/second. Thus, the system throughput can also be increased.
While hologon scanning systems provide advantages over the drum scanner described above, they also have inherent disadvantages. It is difficult to increase system throughput by providing multiple write beams from multiple laser sources in hologon scanners, for example by the use of a laser diode array, because hologons are very sensitive to variations in the wavelength of the multiple write beams. The grating employed in hologon scanners is matched to a specific wavelength. Minor variations in wavelength between the beams, for example fractions of a nanometer, results in deflection errors and a degradation in the reproduced image. It is extremely difficult, however, to produce and maintain an array of laser diodes to have precisely matched wavelength characteristics in order to avoid this problem. In addition, early hologon scanners having a grating disc mounted on a motor shaft were susceptible to "wobble", i.e. the movement of the grating disc caused by bearing inaccuracies or other mechanical factors, which induces error in the deflected light beams. The grating discs are also susceptible to the formation of tiny cracks around their mounting point to the motor shaft due to vibrations when rotated at high speeds.
Efforts have been made to produce hologon scanning systems that are not susceptible to disadvantages set forth above. For example, Japanese Kokai No. 59-101068 proposes to eliminate the problems associated with the mounting of the grating disc on the motor drive shaft by making the disc part of the rotor of the motor. Correction for wobble by making the angle of incidence equal the diffraction angle is discussed in U.S. Pat. No. 4,289,371 issued to Kramer. However, while improvements have been made to correct for wobble and the cracking of the grating discs, the hologon scanning systems are still susceptible to error induced by laser sources mismatched in wavelength. Thus, it would be desirable to provide a laser scanning system of compact design that is wobble free and insensitive to minor variations in wavelength between multiple light sources.